Coherent optical radar processor

ABSTRACT

A radar processing system and method for extracting from received radar data information about moving targets even though the absolute energy reflected by these targets is much less than the energy reflected by the stationary background. The processing system includes an optical recorder for recording the magnitude of the reflected energy and a coherent optical processor for processing the recorded information by computing the power spectrum of the Doppler frequency shift in the recorded information and thereby distinguishing a true target from the stationary background. In addition, means are provided for eliminating from the recorded information any range ambiguities produced at higher pulse repetition frequencies because, at these higher frequencies, echoes from far ranges return at the same time as echoes generated by later radar pulses return from near ranges. The optical processor can operate on data from airborne or groundbased radars.

United States Patent [191 Thomas COHERENT OPTICAL RADAR PROCESSOR [75]Inventor: Carlton E. Thomas, Van Nuys,

Calif.

[73] Assignee: KMS Industries, Inc., Ann Arbor,

Mich.

[22] Filed: May 25, 1970 [21] Appl. No.: 40,265

[52] US. Cl 343/5 PC, 343/6 R, 343/7.7 [51] Int. Cl. G0ls 9/42 [58]Field of Search 343/5 PC, 6 R, 7.7

[56] References Cited UNITED STATES PATENTS 3,127,605 3/1964 Alderson343/7] 3,066,289 11/1962 Elbin'ger 343/7] 3,127,607 3/1964 Dickey, Jr...343/7.7 X 3,110,023 11/1963 Chaffee 343/7] X Primary Examiner-Malcolm F.l-lubler Attorney, Agent, or FirmBarnes, Kisselle, Raisch & ChoateANALYZER [5 7] ABSTRACT A radar processing system and method forextracting from received radar data information about moving targetseven though the absolute energy reflected by these targets is much lessthan the energy reflected by the stationary background. The processingsystem includes an optical recorder for recording the magnitude of thereflected energy and] a coherent optical processor for processing therecorded information by computing the power spectrum of the Dopplerfrequency shift in the recorded information and thereby distinguishing atrue target from the stationary background. In addition, means areprovided for eliminating from the recorded information any rangeambiguities produced at higher pulse repetition frequencies because, atthese higher frequencies, echoes from far ranges return at the same timeas echoes generated by later radar pulses return from near ranges. Theoptical processor can operate on data from airborne or groundbasedradars.

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' iOO PATENTED JIWZZIQM 3. 787 8 42 SHEEI t Of 4 MONOCHROMATIC LIGHTRANGE AZIMUTH H COHERENT OPTICAL RADAR PROCESSOR BACKGROUND OF THEINVENTION 1. Field of the Invention This invention relates to radarprocessing systems and more particularly to a coherent opticalprocessing system whereby moving targets may be readily distinguishedfrom background clutter.

2. Description of the Prior Art Since many radars operate with morebackground noise than target signals, the detection of moving targets inthis background noise has been a long standing problem in the radar art.At any instant of time, the electromagnetic energy detected by a radarreceiver due to the background may exceed the energy from targets byfour or five orders of magnitude. The usual display techniques would notreveal any targets since they would be hidden in the background clutternoise. Some means of post detection processing is thus required todetect targets in the presence of high background clutter.

One common method of such processing discriminates targets from clutteron the basis of target motion, since the target motion causes a Dopplershift in the frequency of the reflected energy, whereby the stationaryground or clutter frequencies are not shifted significantly. A movingtarget indicator (MTI) or airborne moving target indicator (AMTI) maythus distinguish between moving targets and the stationary background bymeasuring the Doppler frequency shift of all returned echoes and passingonly the shifted frequencies to the output display.

There are two general types of MTI processors employing the Dopplerfrequency shift principle in operation today; delay line cancellers andpulse doppler processors. A delay line canceller operates on the phaseshifts produced by moving targets. lfa short radar pulse illuminates amoving target at one instant of time, the reflected energy will have acertain phase relationship relative to the local oscillator of theradar. Fractions of a second later, a second radar, pulse illuminatesthe same target. If the target has moved in this interval, the reflectedenergy will have a slightly different phase shift from the first echo.Stationary targets, on the other hand, produce exactly the same. phaseshift, pulse after pulse for a ground based radar. In employing thistype processor, the radar electronics compare the phase of the reflectedechoes with the phase of a coherent local oscillator. Since the phase ofsignals reflected from moving targets changes from one pulse to thenext, a moving target generates a varying output from the radarelectronics whereas successive pulses from a stationary target do notcause such a change. Thus, if alternate pulses are subtracted, thebackground information is cancelled out. To implement the subtraction ofalternate pulses, each pulse may be stored and delayed until the nextpulse returns from the target. The delay line stores and delays theradar waveform, and conventional subtraction circuits cancel thestationary background clutter.

While delay line canceller radars are effective for some applications,they are limited in the amount of clutter which they can cancel.Backgrounds which fluctuate with time (trees, seas,rain, etc.) andsystem instabilities can generate the equivalent of moving clutter andtherefore cause false alarms. Moreover, for AMTI radars, the backgroundis always in motion relative to the radar since the radar is moving.Delay line canceller AMTI systems do work to some degree, but they donot provide adequate clutter cancellation over several classes ofbackgrounds, particularly cities and other man-made structures. A secondproblem with delay line cancellers is the amount of equipment stabilityrequired. The entire radar system must be exceptionally stable toprevent equipment changes from altering the phase or amplitude of theradar signal. Such a degree of stability is difficult to achievepractically; for example, a delay line is normally very sensitive totemperature variations.

A second general type of operational MTI radar is the pulse Dopplerradar. In this type radar, the detected radar signal is first separatedby a :set of range gates, so that data from a given range intervalalways passes through the same range gate on successive radar pulses.Once the data signals are segregated into range intervals, the signalsderived from moving targets fluctuate and are passed by appropriateelectrical filters which reject the stationary clutter. Although thistype radar achieves improved clutter cancellation, its major difficultyis its complexity, since it requires one range gate for each rangeresolution element. For many practical applications, radars must have10,000 or more such range elements. Even with modern micro-circuittechnology, the production of 10,000 discrete circuits could beprohibitive. As a result, most practical Doppler radars have beenlimited to hundreds of range gates or less.

SUMMARY OF THE INVENTION This invention provides a coherent opticalradar processing system whereby information about moving targets may beextracted in the presence of high background clutter. The processingsystem includes an optical recorder for recording the radar data and anoptical processor for processing this stored record in such a way thatmoving targets are distinguished from the background clutter. Theoptical recorder includes an electro-optical modulator which modulates acoherent light beam in accordance with the radar echoes received fromtransmitted radar pulses. This modulated beam is focused upon aphotosensitive film which comprises a recording medium. The film isadvanced as the modulated beam scans across it in a direction transverseto the direction of its advancing motion. The scanning is synchronizedwith the transmitted radar pulses so that the echoes from one pulse arerecorded on the film in a direction transverse to its advancing motion.The echoes from the next transmitted pulse are then similarly recordedso that the echo waveforms (reflected energy versus range for eachpulse) are recorded side-by-side on the film.

The coherent optical processor then operates upon this recorded radardata in such a fashion that moving targets may be distinguished from anybackground clutter. The record film is advanced through a beam ofcollimated, coherent light. A lens system then focuses the light beamwhich has been modulated by the entry of the stored radar data. The lenssystem is constructed such that the Doppler shift of the radar datarecorded in the direction of advancing motion of the record filmdetermines one directional displacement of the image and the radar datarecorded in the transverse direction of motion determines the otherdirectional displacement of the image. In an AMTI system, additionalmeans are used to cancel that portion of the image which representsbackground information; however, this can be easily accomplished sincethe Doppler shift of a stationary background may be calculated and thisportion of the image may then be removed;

Moreover, the system may be adapted to remove range ambiguitiesresulting from high pulse repetition frequencies since, at thesefrequencies, echoes from far ranges return at the same time as echoesgenerated by later pulses returned from near ranges. In this system, thesynchronization of the scanning means and the radar pulses is such thatthe radar data from the minimum radar range always starts at the bottomof the film and the data from the first ambiguous range always falls atthe top of the film, where the first ambiguous range is defined as thatrange whose echo will coincide with the echo from the minimum range whenit is illuminated by the next pulse. Furthermore, the time betweentransmitted radar pulses is altered according to the progression:

where z is the time between the first and second radar pulse and n is'apredetermined number; the progression being repeated once it has beencompleted. Thus, all the recorded radar data derived from a given rangeambiguity falls along a line of a predetermined angle on the film. Amultichannel optical processor then processes the stored data, eachchannel operating along a predetermined angle in a similar fashion asthe single channel processor described above.

A general object of the invention is to provide an improved radarprocessing system for extracting information from radar data aboutmoving targets even though the absolute energy reflected by thesetargets is much less than the energy reflected by the stationarybackgrounds.

Another object of the invention is to provide a radar processing systemwith a large storage capacity and a high data processing rate.

A further object of the invention is to provide a radar processingsystem which provides a permanent record of receivedradar data-which canbe reprocessed for further analysis.

., An additional object of the invention is to provide a coherentoptical radar processing system.

Another object of the invention is to provide a radar processing systemwhich has means for resolving range ambiguities produced at higher pulserepetition frequency rates.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates an optical recorderemployed in the invention;

FIG. 2 illustrates a transfer function of an electrooptic modulator;

FIG. 3 illustrates a feedback loop employed to linearize the output ofan electro-optic light modulator;

FIG. 4 is a block diagram of a closed loop mirror synchronizationsystem;

FIG. 5 illustrates the Fourier transforming properties of sphericallenses in coherent light;

FIGS. 6a and 6b illustrate a lens system producing a one dimensionalFourier transform;

FIGS. 7a and 7b illustrate the focusing properties of a 'zone plate incoherent light; 1

FIG. 8 illustrates an idealized film record of two moving targets asgenerated by the optical recorder of the present invention;

FIGS. 9a and 9b are schematic views of an optical processor employed inthe present invention;

FIG. 10 illustrates an output plane of the optical processor of FIG. 9;

FIG. 11 illustrates the pulse repetition pattern employed to resolverange ambiguities in the subject invention; and

FIG. 12 illustrates a target history format on the record film whenemploying the pulse repetition pattern illustrated in FIG. 11.

DESCRIPTION OF THE PREFERRED EMBODIMENT The following descriptionsrelate to an optical processor as applied to an airborne radar. Sincethe airborne application is technically more difficult than the groundbased radar application, the extension to the latter will be obvious toone skilled in the art. Two distinct subsystems are required for theoptical processing system; an optical recorder and an optical processor.

The purpose of the recorder is to store the radar data on some substrateso that it may be processed by the optical processor. The data may bestored in a form of intensity variations or phase variations on thefilm. Two general types of recorders are feasible; a cathode rayrecorder and a laser recorder. The laser recorder has the advantage ofbrighter spot irradiance which allows the use of lower sensitivity filmswhich, in turn, have higher resolution and less spatial noise.Therefore, the following descriptions relate to a laser recorderalthough a cathode ray tube recorder may also be employed.

FIG. 1 is a schematic illustration of an optical recorder. A laser 10provides a collimated laser beam which enters a light modulator 12. Themodulator 12 may be of any conventional design although an electroopticmodulator is preferred. It is coupled to the received radar signal andmodulates the polarization of the laser beam in proportion to thisreceived signal. An analyser l4 converts this polarization modulationinto intensity modulation. A beam expander 15 comprises a short focallength lens followed by a long focal length lens, with asmall pinholeplaced at the focus of the first lens, is used to remove spatial noisefrom the laser beam and thereby increase recording resolution. Arotating mirror 16 comprises a scanning means and scans this modulatedlight beam rapidly across the width of the recording medium 18, a stripof photosensitive film. The lens 20 focuses the beam onto the film. Therotating mirror 16 is synchronized to the radar in such a manner thatthe laser beam is at the bottom edge of the film at the instant a radarecho returns from the closest range interval. The mirror rotation rateis controlled by the rotating mirror assembly 22 and for the followingdescriptions it is assumed that the rotation rate is fixed on the film,scan by scan. The capstan 24 is made to move very uniformly at avelocity proportional to the pulse repetition frequency of the radar.

Several features of this laser recorder should be emphasized. A first isthe use of a beveled scanning mirror 16 as illustrated. Conventionalscanners normally use polygon mirrors which necessarily limit theproximity of the polygon to the focusing lens. For the presentapplication, the spacing between the scanning mirror and this lensideally should be zero so as to reduce the required lens aperture size.Thus, with a 45 beveled polygon mirror as illustrated, the mirror can bemoved much closer to the lens.

A second feature relates to the light modulator and is necessary to makethe operation of the system practical. A transfer function of a typicalelectro-optic modulator approximates a cosine curve as illustrated inFIG. 2. This non-linear response cannot be tolerated in an AMTI radar,since the ground clutter would be nonlinearly recorded. This would inturn produce harmonics of the ground clutter spectra which, if they fellinto the regions of the Doppler frequency band normally occupied only bymoving targets, would produce false targets as the harmonics of theclutter would be misinterpreted as moving targets. Furthermore, thelight output from a laser is not perfectly uniform and fluctuates withtime. This laser noise would be recorded on the film along with theradar information.

Both of these deficiencies can be significantly reduced by employing afeedback loop as illustrated in FIG. 3. The beam splitter 30, thephotomultiplier 32, and the amplifier 34 monitor the light fluctuationsat the output of the modulator which is compared by the summing node 36with the desired radar video signal. An error signal representing thedifference between these signals is generated on line 38 and thenamplified by the drive amplifier 40. This amplified signal is thenemployed to modulate the light modulator 42. It can be shown byclassical feedback theory that the modulator transfer function ispartially linearized and the laser noise effectively cancelled by thisfeedback system.

The third important feature is the mirror synchronization device. Asdescribed above, the scanning mirror must be synchronized with the radarpulses so that the echo wave-forms (reflected energy versus range foreach pulse) are recorded side-by-side on the recording film. Therefore,the energy from the nearest range for a first pulse must be in parallelon the film with the energy reflected from the same range on allsuccessive pulses. There are two basic approaches to the synchronizationof the mirror; a closed-loop slave operation and an open selfsynchronousoperation.

A block diagram for a closed loop synchronization system is illustratedin FIG. 4. The position of the mechanically scanned laser beam ismonitored by a stationary slit and light detector assembly 50. As eachlaser beam scans across the slit, an electrical pulse is generated. Thepulse train so produced by the laser beam is compared with the radartrigger pulse train by the pulse comparison network 52. This networkgenerates an error signal representing the phase difference betweenthese two pulse trains. The error signal then is amplified in the driveamplifier 54 which is connected to the scanning mirror motor 56 in sucha fashion to either advance or retard the rotation rate of the scanningmirror depending upon the magnitude and polarity of the generated errorsignal. Thus, the loop tends to bring the pulse train produced by theslit and light detector into coincidence with the radar trigger pulsetrain thereby slaving the mirror rotation to the radar pulse repetitionfrequency.

Such a closed loop mirror synchronization system. however, places astringent manufacturing requirement on both the laser recordercomponents as well as the trigger generation circuits in the radar. Inparticular, the beveled polygon mirror facets must be manufactured tovery close tolerances.

The preferred synchronization method, therefore, is an open-loop,self-synchronous operation. When employing this approach, the pulsetrain produced by the slit and light detector is used to directlytrigger the radar. The optical system thus performs the system triggergeneration function and eliminates the need for a correspondingelectronic trigger generation circuit. Such an approach assuressynchronization between the radar pulses and the rotation rate of thescanning mirror.

The operation of the optical processor will now be discussed. Theprimary purpose of the optical processor is to compute the Fouriertransform or the power spectrum of the Doppler frequency data recordedon the recording film. The moving targets may thereby be discriminatedfrom the ground clutter background. A second function of the opticalprocessor is to improve the range resolution of a chirped radar bycompressing the received echoes. As it will be :seen, both the spectrumanalysis and pulse compression operations are easily performed by acoherent optical system.

Such a spectrum analysis operation is inherent in a coherent opticalsystem as illustrated in FIG. 5. The light amplitude distribution 58 atthe back focal length of a coherently illuminated lens 60 isproportional to the two dimensional Fourier transformation of any datawritten on a film 62. Thus, if a function g (x,y) is recorded on thefilm the light distribution G(w,, m is formed according to the equation:

t t z. y) ffgbcw dxlly Such an optical Fourier transform function isdiscussed in Principles of Optics, Born and Wolf, Pergamon Press, 1959,Sections 8.3.3 and 8.6.1, and see especially Equation 43 on Page 385. Inthis reference, the Fourier transform is referred to as the Fraunhoferdiffraction pattern.

For an AMTI processor, however, only a one dimensional spectrum analysisis required. This reduction to only one dimension is achieved by addinga cylindrical lens as illustrated in FIG. 6. A cylindrical lens 66 isintroduced between-the film 62 and the spherical lens 60. FIG. 6aillustrates-the top view of such a lens system. In this perspective, thecylindrical lens has no power and behaves merely as a block of glass.Thus, the spherical lens 60 images the Fourier transform onto the outputscreen 68 as discussed above. In the side view, as

illustrated in FIG. 61;, however, the cylindrical lens 66 has magnifyingpower. When it is placed one focal length from the film, horizontallines recorded on the film are imaged into horizontal lines: in theoutput plane 68: The light variations along each horizontal line in theoutput plane, however, remain proportional to the power spectrum of thesignal on the corresponding horof adding a cylindrical lens is achannelized or one dimensional spectrum analysis.

The pulse compression operation is even more fundamental and requires noadditional optics. In a chirped radar, the received echo waveformsproduce a zone plate pattern on the recording film as illustrated inFIG. 7(a). Such zone plate patterns having focusing proper ties similarto a lens. A one dimensional zone plate is illustrated in FIG. 7(a); theshaded areas are ideally opaque and the clear areas are transparent. Thefrequency increases linearly from left to right. It should be noted thatthe zone plate pattern illustrated is a square wave zone plate, that is,the density changes abruptly from one zone plate to the next. In actualradar practice, the modulation would be more nearly sinusoidal with agradual change from the transparent to an opaque region. A side view ofthis zone plate placed in a collimated, coherent light beam isillustrated in FIG. 7(b). The'light rays passing through the zone plate70 are defracted so that they focus to a-spot 72 as illustrated. Thefocal lengthfof a zone plate may be calculated by assuming that thelight passing through each zone must add in phase at the focal spot. Thepath length from the nth zone to the focal spot must therefore differfrom the n-l zone by one wavelength of light. From the geometry of, FIG.7(b), this requires that X +j =f 2n)\f+ n k and assuming that N is verysmall, this equation may be reduced as:

These zone plates actually generate three beams (undefracted,converging, and diverging) and the processor must block two of thesethree beams.

An idealized representation of an AMTI radar signal film appears in FIG.8. The record represents the pattern from two point targets without anybackground clutter. The vertical position of the data is approximatelyproportional to the range of the targets and the horizontal position isproportional to the azimuth angle of the target. Each pattern has adistinct, sinusoidal modulation (shown as a square wave in the figure)in the azimuth direction. The spatial frequency of this patternis.proportional to the Doppler frequency shift produced by each target.Thus the target generating the pattern on the upper left has a higherradial velocity (higher grating frequency) than the other target.

The vertical modulation for each pattern is identical and produces aFresnel zone plate type of pattern. This pattern results from the linearfrequency shift in the transmitted radar pulse of a chirped radarsystem, thereby allowing the use of a longer pulse with lower peak powerrequired for a given-range resolution requirement.

To be detected as targets, the optical processor must convert thepatterns illustrated in FIG. 8 into point spots of light. Therefore, itmust simultaneously focus the one dimensional zone plates and the onedimensional Doppler grating. The horizontal and vertical lines of thepatterns illustrated in FIG. 8 represent the boundaries of alternateopaque and transparent areas. A schematic illustration of an opticalprocessor which achieves these functions, is illustrated in FIG. 9. Thesystem is similar to the channelized spectrum analyzer of-FIG. 6 withthe following exceptions:

I. Since the zone plates on-the film 80 focus part of the coherent lightenergy to a line 82 in front of the film, the front focal length of thecylindrical lens 84 is placed in coincidence with the focal plane 82 ofthe vertical zone plates. The cylindrical lens, in conjunction withspherical lens 88, images this line to the output plane 86.

2. The separation between the cylindrical lens 84 and the spherical lens88 is increased. This increased separation does not alter the optics ofthe system since for a channelized analyzer this distance is arbitraryand is normally minimized. This increased separation allows a DC block90 to be inserted in the lens system at the back focal length of thecylindrical lens 84. The DC block 90 functions to eliminate the DCvirtual images from each zone plate and is effective regardless of thelocation of the zone plates on the input film.

The output from the optical processor is a two dimensional lightpattern. For the idealized case of two point targets in the radar fieldof view wherein the record film resembles that illustrated in FIG. 8,these two target signals .would be focused to two distinct points oflight in the processor output plane 86, as illustrated in FIG. 10. Thevertical displacement of any spot of light indicates the range of'thecorresponding target and the horizontal displacement of a spot indicatesthe radial velocity of the target with respect to the radar. Thus, spot92 represents a target at a near range of 'R1 and spot 94 indicates atarget at range R2. The target indicated by spot 94 is traveling at afaster radial velocity since its horizontal displacement is greater thanthat of spot 92.

In practice, however, all background clutter reflectors as well as twotargets produce a point of light at the output of the processor. Thepoint of light representing targets are therefore distinguished fromthose representing ground clutter. This is accomplished since the targetDoppler frequency shifts differ from the major ground clutter spectorand thus occur in a different physical location on the output plane ofthe processor. There are several suitable techniques for reading out theprocessed light intensity distribution as well as eliminating thebackground clutter at the sametime. The most-straightforward is to scanthe two dimensional output plane with a TV vidicon tube. Assuming thescan lines are horizontal, each video sweep would contain data from onlyone range interval. Within a single sweep, the independent variable(time) would be proportional to Doppler frequency shifts. Low radialvelocity targets would thus occur early in the TV sweep period whilefaster targets would occur later.

Considering the removal of background clutter, the background clutterfor an AMTIradar could occur anywhere in the output planedepending uponthe antenna angle of the radar transmitter. Assuming the antenna ispointed at an angle 0 with respect to the ground track, the clutterspectrum would be located at a Doppler frequency given by:

distance from the center of an oscilloscope with the angle being equalto the antenna pointing angle. For many radar applications, the opticalprocessor generates too much data for the above vidicon type readout.For example, with one hundred lines per millimeter resolution and inchfilm, over 10,000 range resolution elements would be required. Nopresent vidicon can handle this much data since the very best vidicon/crt displays have less than 4,000 TV lines.

Another method of output detection is to employ an array of lightdetectors and a rotating mirror to scan the processor output plane. Ifthe scanning mirror rotates in a vertical plane, a narrow horizontalslit, for example, microns, may be used to transmit the recorded data. Afiber optic bundle may be placed in the slit, e.g., the slit couldcontain fiber optic elements along the horizontal slit. The fiber opticbundle is used to transmit the vertical, closely spaced Dopplerfrequency shift channels to larger discrete light detectors. Thus, asthe mirror rotates in a vertical plane, each light detector interceptsonly one Doppler frequency band but sequentially samples every rangeinterval. The output from this type of scanner detector is a set ofsignals, each signal having a video waveform which varies with time asthe radar reflectivity varies with range, each signal containing theenergy from only one relatively narrow Doppler frequency band. Thismethod of output detection permits better target discrimination as wellas better height finding capability. When employing this type of outputdetector, the attenuation of the ground clutter may be achieved withvarious techniques.

A first technique is called preprogrammed clutter attenuation.Basically, this method provides for attenuating those Doppler frequencychannels which are known to have a large probability for strong clutternoise because of the angle of the radar antenna. The Doppler frequencyshift for any ground reflector is given by:

f ZVa/A cos 6 where Va aircraft velocity, A radar wavelength, and

the angle between the aircraft ground track and the ground reflector ofinterest. I

If the radar antenna is pointing at the angle away from the groundtrack, then most of the clutter energy will be distributed about thefrequency f as given by the above equation. As the antenna rotates, 6changes and so doesfi, the center frequency of the clutter Dopplerspread. Thus, by monitoring the angle of the radar antenna, the channelcorresponding to the center frequency f may be attenuated, therebyeliminating the background clutter. It should be noted, however, thatthis technique only applies to AMTl radar since the average clutterfrequency from ground based radars is assumed to be zero at all times.

There are several disadvantages to the preprogrammed clutter attenuationtechnique described above. For example, if the background reflectancedecreases, the system continues to block those frequency bands whichmight have clutter and consequently targets which could be detected aretherefore lost. Thus, it would be desirable to have a variable clutterattenuation which adapts to the existing clutter level. Furthermore, ifthe clutter spectrum shifts (as in sea clutter with a water currentchanging direction as a function of range), the clutter attenuationfrequencies should be shifted accordingly.

One method of achieving such an adaptive clutter attenuation is toadjust the gain setting of each Doppler channel according to a weightedaverage of the clutter that existed previously in the same channel andpossibly in adjacent channels. An integrator circuit could average theclutter over several preceding range intervals, and as the clutter levelincreases, the gain is decreased. In addition, a threshold circuit couldbe placed in the Doppler channel which could increase the thresholdsignal required if the other level increases and vice versa. Thesensitivity of a Doppler channel is therefore reduced and the falsealarm probability is kept rather low. When the background reflectivitydecreases as for sea backgrounds or beyond the horizon echoes, thesensitivity increases and targets which would have been missed are nowdetected.

In addition, a post processing digital computer may be used to keeptrack of all moving targets independent of the type of short termclutter attenuation used. The computer could generate a scan-to-scantarget tracking and thus remove any false alarms due to stationarydiscrete clutter echoes which pass through the clutter attenuationcircuits. For the airborne case, discrete ground clutter echoes move inthe direction of the aircraft ground track. The computer can identifystationary targets by comparing the known aircraft ground ve locity withthe tracks of all targets. Any targets which have a velocity equal tothe ground velocity of the radar are rejected by the computer.

The above descriptions have assumed that the pulse repetition frequencyof the radar system is of such a magnitude that range ambiguities arenot produced. Range ambiguities arise at higher pulse repetitionfrequencies since the echoes from far ranges return at the same time asechoes generated by later pulses return from near ranges. At such higherfrequencies, the system described above may be easily modified toresolve any range ambiguities.

The optical recorder optically operates as described above. The onlydifference occurs in the synchronization of the scanning mirror sincethe pulse repetition frequency is now varied according to theprogression t, t+A, t+2A, t+nA, where t is the time spacing between thefirst and second pulses, A is a fixed time, and n is a predeterminednumber. The progression is repeated once it has been completed, asillustrated in FIG. 11. The speed and synchronization of the scanningmirror is such that the data from a minimum radar range always falls atthe bottom of the recording film while the data from the first ambiguousrange always falls at the top of the recording film. The first ambiguousrange is defined as that range whose echo will coincide with the echofrom the minimum range when it is illuminated by the next pulse. Theecho from all farther ranges will be recorded in subsequent sweeps ofthe scanning mirror.

This method of synchronization in confluence with the particular pulserepetition pattern has the'unique feature of causing all targethistories from targets of a given range ambiguity interval, i.e., alltargets between the nth and the nth l ambiguous ranges, to fall alonglines of a predetermined angle on the record film. Thus, as illustratedin FIG. 12, the data from all targets in the near (unambiguous) rangeinterval. fall on horizontal lines, e.g., line 100. The histories oftargets in the first ambiguous range interval all fall along lines ofslope 6, e.g. line 102. Target histories from the nth interval occur onlines of approximate slope n0.

To process the radar data so recorded, a multichannel processor isrequired. Each channel of the processor operates as the single processordescribed above and is used to process the radar data from only oneambiguous range interval, that is, along a fixed angle of the recordfilm. The processors may be placed in parallel, each observing aslightly different part of the signal film. To achieve processing at thedifferent angles, each processor has a cylindrical lens oriented at aunique angle corresponding to the angle on the-film; each processorwould therefore integrate targets from only one ambiguous rangeinterval. Since the record film is moving continuously, each processorcovers all of the azimuth data on the film.

As in the above single channel processor, some method of detection isrequired. Preferably, the detection system would also include means forattenuating the background clutter.

While the invention has been particularly shown and described withreference to the preferred embodiments thereof, it will be understoodthat various changes in form anddetail may be made therein withoutdeparting from the spirit and scope of the invention.

What is'claimed is:

1. In a coherent radar system having means for transmitting radar pulsesand receiving radar echoes from said pulses wherein said received echoesrepresent radar data, a radar processing system for extractinginformation from said radar dataabout moving targets in the presence ofhigh background clutter comprising:

a. optical recording means sensitive to said radar echoes for storingsaid radar data'and comprising:

1. means for generating an optical beam,

2. optical modulating means in the path of the optical beam andresponsive to the radar echoes for modulating the beam, and r 3. anoptical recording medium sensitive to the modulated beam for recordingsaid radar data; and

b. optical processing means for processing said stored data in such away to emphasize said moving targets over said background clutter,whereby said moving targets are distinguished from said backgroundclutter, said optical processing means including optical means forproducing an optical display of the Fourier transform of the storedradar data.

2. The radar processing system of claim 1 wherein said optical beam is acoherent light beam, and wherein said optical recording means comprises:

a. driving means for advancing said optical recording medium; and

b. scanning meansfor scanning said modulated beam across said recordingmedium in a direction transverse to its driven direction, whereby saidradar data is stored on said optical recording medium.

3. The optical recording means of claim 2'further comprising:

a. comparator means coupled between the output of said opticalmodulating means and said received radar echoes for generating an outputsignal representing the difference between these signals, and

b. voltage driving means coupled to said optical modulating means andsensitive to said output signal for controlling said optical modulatingmeans whereby the output of said modulating means is linearized. 4. Theoptical recording means of claim 2 wherein said scanning means comprisesa beveled scanning mir- 5 ror.

5. The optical recording means of claim 2 further comprising:

a. timing means coupled to said scanning means and said radar pulses forsynchronizing said scanning means with said radar pulses, whereby theradar data may'be recorded in a predetermined manner on said opticalrecording medium.

6. The radar processing system of claim 1 wherein:

a. said radar data is stored in vertical and horizontal directions, theradar data in said vertical direction representing the radar echoes fromone radar pulse while traveling from the nearest to the farthest range,and the radar data in the horizontal direction representing similar datafrom successive 2O radar pulses; and

b. said optical means produces from said stored data the Fourierspectrum of the Doppler frequency shift in the horizontally stored radardata. 7. The radar processing system of claim 6 wherein said opticalprocessor comprises:

a. a source of collimated, coherent light for producing a beam of light;

b. driving means for advancing said stored radar data through said beamof light in a horizontal direction, whereby said radar data modulatessaid beam;

0. a lens system placed in said beam after the entry of said radar datafor focusing said modulated beam into images; whereby the Doppler shiftof said horizontally recorded data determines one directionaldisplacement of said displayed image and said vertically recorded datadetermines the other directional displacement of said displayed image.

8. The optical processor of claim 7 further comprising detector meansfor attenuating said images derived from said background clutter.

9. In a coherent radar system having means for transmitting radar pulsesand receiving radar echoes from said pulses wherein said received echoesrepresent radar data, said radar pulses being transmitted at such afrequency that radar echoes from several pulses may be received in thesame time interval thereby creating range ambiguities in the radar dataas to these echoes, a radar processing system for resolving said rangeambiguities comprising:

a. an optical recorder for storing said radar data, said storage takingplace in vertical and horizontal directions, the radar data in saidvertical direction representing the radar echoes from one radar pulsewhile traveling from a minimum radar range to a first ambiguous range,and the radar data in the horizontal direction representing similar datafrom successive radar pulses;

b. timing means for varying the time period between successive radarpulses according to the progression t, t+A, t+2A, t+nA, where t is thetime between the first and second pulses, A is a fixed time interval,and n is a predetermined number, said progression being repeated oncecompleted, whereby all stored radar data of a given range ambiguityfallsalong a line at apredetermined angle; and

c. a multichannel optical processor for processing said stored data,whereby each channel .processes one ambiguous range interval, saidoptical processor including optical means for producing an opticaldisplay of the Fourier transform of the stored radar data.

10. In a coherent radar system having means for transmitting radarpulses and receiving radar echoes from said pulses wherein said receivedechoes represent radar data, a method for extracting information fromsaid radar data about moving targets in the presence of high backgroundclutter comprising the steps of:

a. optically storing said radar echoes by generating a recording opticalbeam, modulating the beam in proportion to the received radar echoes.and recording the modulated beam on an optical recording medium; and

b. optically processing said optically recorded data in such a way toemphasize said moving targets over said background clutter, whereby saidmoving targets are distinguished from said background clutter, saidprocessing step including optically producing the Fourier transform ofthe recorded data. 11. The method of claim 10 wherein the recording beamis a coherent light beam, and wherein said step of optically recordingcomprises the steps of:

a. advancing the optical recording medium; and

b. scanning said modulated recording beam across said recording mediumin a direction transverse to its driven direction, whereby said radardata is stored on said optical recording medium.

12. The method of claim 10 wherein said optical processing stepcomprises the steps of:

step of attenuating those images derived from said background clutter.

1. In a coherent radar system having means for transmitting radar pulsesand receiving radar echoes from said pulses wherein said received echoesrepresent radar data, a radar processing system for extractinginformation from said radar data about moving targets in the presence ofhigh background clutter comprising: a. optical recording means sensitiveto said radar echoes for storing said radar data and comprising: 1.means for generating an optical beam,
 2. optical modulating means in thepath of the optical beam and responsive to the radar echoes formodulating the beam, and
 3. an optical recording medium sensitive to themodulated beam for recording said radar data; and b. optical processingmeans for processing said stored data in such a way to emphasize saidmoving targets over said background clutter, whereby said moving targetsare distinguished from said background clutter, said optical processingmeans including optical means for producing an optical display of theFourier transform of the stored radar data.
 2. optical modulating meansin the path of the optical beam and responsive to the radar echoes formodulating the beam, and
 2. The radar processing system of claim 1wherein said optical beam is a coherent light beam, and wherein saidoptical recording means comprises: a. driving means for advancing saidoptical recording medium; and b. scanning means for scanning saidmodulated beam across said recording medium in a direction transverse toits driven direction, whereby said radar data is stored on said opticalrecording medium.
 3. The optical recording means of claim 2 furthercomprising: a. comparator means coupled between the output of saidoptical modulating means and said received radar echoes for generatingan output signal representing the difference between these signals, andb. voltage driving means coupled to said optical modulating means andsensitive to said output signal for controlling said optical modulatingmeans whereby the output of said modulating means is linearized.
 3. anoptical recording medium sensitive to the modulated beam for recordingsaid radar data; and b. optical processing means for processing saidstored data in such a way to emphasize said moving targets over saidbackground clutter, whereby said moving targets are distinguished fromsaid background clutter, said optical processing means including opticalmeans for producing an optical display of the Fourier transform of thestored radar data.
 4. The optical recording means of claim 2 whereinsaid scanning means comprises a beveled scanning mirror.
 5. The opticalrecording means of claim 2 further comprising: a. timing means coupledto said scanning means and said radar pulses for synchronizing saidscanning means with said radar pulses, whereby the radar data may berecorded in a predetermined manner on said optical recording medium. 6.The radar processing system of claim 1 wherein: a. said radar data isstored in vertical and horizontal directions, the radar data in saidvertical direction representing the radar echoes from one radar pulsewhile traveling from the nearest to the farthesT range, and the radardata in the horizontal direction representing similar data fromsuccessive radar pulses; and b. said optical means produces from saidstored data the Fourier spectrum of the Doppler frequency shift in thehorizontally stored radar data.
 7. The radar processing system of claim6 wherein said optical processor comprises: a. a source of collimated,coherent light for producing a beam of light; b. driving means foradvancing said stored radar data through said beam of light in ahorizontal direction, whereby said radar data modulates said beam; c. alens system placed in said beam after the entry of said radar data forfocusing said modulated beam into images; whereby the Doppler shift ofsaid horizontally recorded data determines one directional displacementof said displayed image and said vertically recorded data determines theother directional displacement of said displayed image.
 8. The opticalprocessor of claim 7 further comprising detector means for attenuatingsaid images derived from said background clutter.
 9. In a coherent radarsystem having means for transmitting radar pulses and receiving radarechoes from said pulses wherein said received echoes represent radardata, said radar pulses being transmitted at such a frequency that radarechoes from several pulses may be received in the same time intervalthereby creating range ambiguities in the radar data as to these echoes,a radar processing system for resolving said range ambiguitiescomprising: a. an optical recorder for storing said radar data, saidstorage taking place in vertical and horizontal directions, the radardata in said vertical direction representing the radar echoes from oneradar pulse while traveling from a minimum radar range to a firstambiguous range, and the radar data in the horizontal directionrepresenting similar data from successive radar pulses; b. timing meansfor varying the time period between successive radar pulses according tothe progression t, t+ Delta , t+2 Delta , . . . t+n Delta , where t isthe time between the first and second pulses, Delta is a fixed timeinterval, and n is a predetermined number, said progression beingrepeated once completed, whereby all stored radar data of a given rangeambiguity falls along a line at a predetermined angle; and c. amultichannel optical processor for processing said stored data, wherebyeach channel processes one ambiguous range interval, said opticalprocessor including optical means for producing an optical display ofthe Fourier transform of the stored radar data.
 10. In a coherent radarsystem having means for transmitting radar pulses and receiving radarechoes from said pulses wherein said received echoes represent radardata, a method for extracting information from said radar data aboutmoving targets in the presence of high background clutter comprising thesteps of: a. optically storing said radar echoes by generating arecording optical beam, modulating the beam in proportion to thereceived radar echoes, and recording the modulated beam on an opticalrecording medium; and b. optically processing said optically recordeddata in such a way to emphasize said moving targets over said backgroundclutter, whereby said moving targets are distinguished from saidbackground clutter, said processing step including optically producingthe Fourier transform of the recorded data.
 11. The method of claim 10wherein the recording beam is a coherent light beam, and wherein saidstep of optically recording comprises the steps of: a. advancing theoptical recording medium; and b. scanning said modulated recording beamacross said recording medium in a direction transverse to its drivendirection, whereby said radar data is stored on said optical recordingmedium.
 12. The method of claim 10 wherein said optical processing stepcomprises the steps of: a. generating a processing beam of collimateD,coherent light; b. advancing said recorded radar data through saidprocessing beam of light, whereby said recorded radar data modulatessaid beam; c. focusing said modulated processing beam into images; andd. optically displaying said images.
 13. The method of claim 12 furthercomprising the step of attenuating those images derived from saidbackground clutter.